Components containing composite systems having at least two flat or curved layers comprising different inorganic, ceramic phases are well known in industry. Examples which may be mentioned are gas sensors, such as are used for the measurement of carbon monoxide in exhaust gases of internal combustion engines, and also heating elements. The composite systems can be produced in a known manner by sintering finely divided inorganic materials in two or more stages, by first shaping and sintering the material which forms the first layer, then applying the material of the second layer onto the first sintered layer and again sintering and repeating this procedure until the desired number of layers has been reached. Owing to the many operations, this process is not optimum in terms of production technology. In addition, the adhesion of the layers not infrequently leaves something to be desired.
More advantageous is another known process in which the finely divided materials which form the various layers of the finished composite system are arranged in layers and sintered together. This process is also described as cosintering, and, in it, the bonding of the various layers is generally better than in the above-described process. Nevertheless, the composite bodies are frequently not completely satisfactory with regard to mechanical and thermal stressability, with thermal stressing being understood to include frequent change between cold and hot operating states. The stresses occurring during this process can lead to the formation of cracks and to detachment of parts of the composite body.
U.S. Pat. No. 4,806,739 discloses ceramic heaters produced by cosintering, in which heaters the heating element is separated by an aluminium oxide layer from a solid electrolyte comprising zirconium(IV) oxide. In column 2, lines 22 to 30, it is stated that a small amount of zirconium(IV) oxide in the aluminium oxide suppresses the tendency to shrink (or the shrinkage) of the layer and stronger bonding of the two layers is achieved.
European Patent 203 351 describes an oxygen sensor produced by cosintering, which sensor contains, between a layer of a solid electrolyte, namely zirconium(IV) oxide, and an electrically insulating layer containing aluminium oxide as main constituent, an intermediate layer which reacts with the solid electrolyte and with the aluminium oxide of the insulating layer and whose linear coefficient of thermal expansion lies between those of the materials in the two layers mentioned (Claim 1). The intermediate layer advantageously comprises zirconium(IV) oxide which is partially stabilized with 6 mol per cent of yttrium oxide and contains 3 per cent by weight of aluminium oxide (column 8, lines 9 to 11). It reduces the stress which results from the different coefficients of thermal expansion of the two layers and which, without the intermediate layer, could destroy the oxygen sensor (column 7, lines 58 to 62). Furthermore, the intermediate layer is supposed to improve the bonding between the two layers, since zirconium(IV) oxide and aluminium oxide hardly react with one another (column 7, lines 62 to 64).
The patent further discloses that an inorganic binder comprising 30 per cent by weight of aluminium oxide, 53 per cent by weight of silicon dioxide and 17 per cent by weight of magnesium oxide is added to the aluminium oxide of the electrically insulating layer (column 6, lines 54 to 60). Another suitable binder contains from 5 to 30 per cent by weight of aluminium oxide and from 70 to 95 per cent by weight of silicon dioxide (column 7, lines 37 to 39). The binder is supposed to melt below the sintering temperature of the aluminium oxide, accelerate the sintering of the aluminium oxide and counteract the shrinkage during cosintering with zirconium(IV) oxide (column 7, lines 20 to 36). At the same place it is stated, without further elaboration, that the shrinkage during sintering can also be counteracted by matching the particle size distribution of the zirconium(IV) oxide and the aluminium oxide.